Temperature-compensated waveguide resonator

ABSTRACT

An evanescent H-mode rectangular waveguide resonator having a tuning screw mounted on one broad wall is temperature compensated by having said screw physically penetrate close to the opposite broad wall of said waveguide. Said penetration provides significant end-loading capacitance. In one preferred embodiment, said screw is offset from the longitudinal centerline of said waveguide to achieve both the required tuning capacitance and the required penetration. In another preferred embodiment, to further improve temperature compensation, a silver-plated Swedish iron screw having a lower coefficient of expansion than the material of the waveguide, (i.e., copper) penetrates close to the opposite broad wall.

United States Patent [72] Inventors Kenneth George Hodgson Loughton; George Frederick Craven, Harlow; Raymond Richard Thomas, Harlow, all of, England [2]] Appl. No. 865,110 [22] Filed Oct. 9, I969 [45] Patented Aug. 24, I971 [73] Assignee International Standard Electric Corporation New York, N.Y.

[54] TEMPERATURE-COMPENSATED WAVEGUIDE RESONATOR 2 Claims, 5 Drawing Figs.

. [52] US. Cl 333/82 BT, 333/83 T,333/73 w [5l] 1nt.Cl H0lp 7/06,

H03n 13/00, HOlp 1/30 [50] Field of Search 333/82 ET, 834, 82,83 T, 73 W, 73

[56] References Cited UNITED STATES PATENTS 2,103,457 12/1937 Hansell 333/82 BT X 2,608,671 8/1952 Fremlin et al.. 333/82 BT X 2,749,523 6/1956 Dishal 333/73 W 2,943,280 6/1960 Brill 333/73 W 3,422,380 l/l969 Kurodaetal. 333/73W OTHER REFERENCES Ragen, G. L., Microwave Transmission Circuits," Mc- Graw Hill, 1948 pp. 489

Craven et al., Design of Microwave Filters With Quarter- Wave Couplings," Proc. IEE (London) Vol. 103 B, pp. L73- l77, 3-l956 Marcuvitz, N., Waveguide Handbook, Mc- Graw-Hill, I951 Primary ExaminerHerman Karl Saalbach Assistant Examiner-Wm. H. Punter Attorneys-C. Cornell Remsen, Jr., Walter J. Baum, Paul W. Hemniinger, Percy P. Lantzy, Philip M. Bolton, Isidore Togut and Charles L. Johnson, Jr.

ABSTRACT: An evanescent H-mode rectangular waveguide resonator having a tuning screw mounted on one broad wall is temperature compensated by having said screw physically penetrate close to the opposite broad wall of said waveguide. Said penetration provides significant end-loading capacitance. In one preferred embodiment, said screw is ofiset from the longitudinal centerline of said waveguide to achieve both the required tuning capacitance and the required penetration. In another preferred embodiment, to further improve temperature compensation, a silver-plated Swedish iron screw having a lower coefficient of expansion than the material of the waveguide, (i.e., copper) penetrates close to the opposite broad wall.

PATENTEB M1824 B71 SHEET 2 0F 2 I nvenlors law/very amoqsm 6501265 F (RAVEN By mono)? moms M Attorney TEMPERATURE-COMPENSATED WAVEGUIDE RESONATOR I BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to temperature compensation of waveguide resonators.

2. Description of the Prior Art I Conventional resonant waveguide cavities are relatively temperature sensitive and where high temperature-frequency stability is necessary, it becomes essential to employ as the manufacturing material a metal with a very low thermal coefficient of expansion, such as a nickel steel invar. This is a relatively expensive material which is also difficult to work. Said disadvantages add significantly to the cost of producing a resonator.

SUMMARY OF THE INVENTION It is an object of the invention to provide an improved 4 evanescent I'I-mode waveguide resonator.

According to the invention there is provided a temperaturecompensated resonator comprising a section of evanescent H- mode waveguide and capacitive screw-tuning means for providing significant end-loading capacitance.

A BRIEF DESCRIPTION OF THE DRAWINGS The above-mentioned and other objects of this invention will become apparent by reference to the following description in conjunction with the accompanying drawings, in which:

FIG. I shows an evanescent mode rectangular waveguide DESCRIPTION OF THE PREFERRED EMBODIMENTS For the purpose of this specification, an evanescent I-I-mode waveguide resonator is defined as a length of rectangular waveguide which has a cutoff frequency above the frequency of the incident I-I-mode and has capacitive tuning screw means extending into the guide from a broad wall of the guide.

Evanescent mode waveguide cavities function on different principles, as described in Waveguide'Band-pass Filters using Evanescent Modes," G. F. Craven, Electronics Letters, Vol. 2, No. 7, July 1966, pp 25-26, and therefore their frequencytemperature dependence is also different.

Referring to FIG. 1, energy present in the dominant H mode in propagating waveguide 1 is transferred to propagating waveguide 2 by evanescent mode cavity 3 having a capacitive tuning screw 4 on the longitudinal centerlines 5 of the guide.

The resonant frequency of an evanescent mode cavity is determined by the conjugate match condition, i.e., the capacitive reactance, of the obstacle in the guide, the screw 4, must be equal, but opposite in sign, to the inductive characteristic of the guide. The resonant frequency, f,, is then given by The resonant frequency is independent, to a first order, of the guide length and merely depends on the cross-sectional dimensions. This, of course, assumes that the value of C, is temperature invariant, an assumption which simplifies the preliminary analysis. The effect ofthe variation of C,, with temperature will be considered later.

The characteristic impedance of waveguide in the H rectangular guide is given by Who/m) where a guide width b guide height A free space wavelength A,, cutoff wavelength 2a A constant In the present application, AIA I and therefore 2,, is imaginary. Equation (l) is derived on this basis and only the value of |'Z,,l is required. Therefore will be dropped from the following mathematical analysis.

Rearranging equation (2) and introducing 8, the fractional change in dimensions resulting fromtemperature change, the resulting characteristic impedance Z, is given by from equation (I) the'modified resonant frequency is given by 7 Assuming that 8 is small and therefore squared and higher power terms can be neglected fjgiciliblh a) And, since )t=V/f where V velocity of light V21rC,,Ab(1+2S;

a a xox. 1{1-.1}]

v tum) Using the binomiel expansion of expand the second term in braces,

Bearing in mind that from equation l) wavelength AA, is in the range This change is a result wholly of the change in the guide width and its effect on the guide characteristic impedance.

The effect of capacitance change on the resonant wavelength is straightforward and evident from equation (la) Ax.,-Ac, 10

The way in which C, changes depends on the type of construction employed. In the construction shown in FIG. I, three effects occur. If the temperature increases,

a. the length of the tuning'screw increases,

b. the guide height increases,

c. the area of the capacitance, i.e., the cross-sectional area of the tuning screw, increases.

If the screw penetration is such that the end of the screw is remote from the opposite (bottom) guide wall, the end loading capacitance is negligible. The effect (a) will then predominate. The normalized reactance of a capacitive screw in waveguide varies in the following way X (1 Cot 6 (l l) where =21rllk (I inserted length of screw).

Thus as 6 increases with temperature the reactance will fall, i.e., the effective value of C, increases. This change will increase the resonant wavelength, i.e., the change in wavelength resulting from the efi'ects on capacitance and inductance will be cumulative.

loading capacitance is significant. The gap, d, between the end of the screw and the bottom wall of the guide then changes to d (1+6) as a result of the difference between the length expansion of the screw and the height expansion of the guide, for an increase in temperature. Since the guide height is the greater, if the material employed in guide and screw is the same, e.g. copper or brass, the gap increases. Considered on its own the capacitance would then tend to decrease, with acorresponding effect on the resonant wavelength. This would offset the effect of temperature on guide dimensions and their consequent effect on resonant wavelength. r

The relationship between the capacitance of a parallel plate capacitor and its various parameters is given by the wellknown expression.

C a (KA/d) where Y K dielectric constant A area d spacing of plates This simple relationship does not hold exactly in determining end-loading capacitance inevanescent waveguide, where d is the gap between the end of the capacitive screw and A is the cross-sectional area of the screw. For example, C. K. Mok (Electronics Letters, Vol. 4, No. 3, 9th Feb. 1968, pp. 43-44) shows that as the width (area) of a capacitive obstacle in evanescent waveguide increases it ultimately becomes an inductance. However, the area of the obstacle has an important effect and consequently this is a parameter which must be considered when the screw size to obtain an optimum penetration is chosen.

The'distance d has to be quite small if the capacitance is to be significant. Obviously, the length of the screw and the guide height will then be nearly identical and, if they are of the same material, the expansions will be nearly identical. However, since the height is always marginally greater its expansion will always be greater and d will always increase. Thus, provided a suitable distance d is chosen and a suitable obstacle width is used, a certain degree of temperature compensation is obtained.

The simplest way in which d can be reduced is by offsetting the screw by a distance x from the guide center towards the guide sidewall, as shown in FIG. 2. The screw penetration will then have to be increased in order to achieve the desired susceptance. In this way an appropriate value of d can be obtained.

An .altemative'arrangeme'nt is shown in FIG. 3, in which there is an olTset main tuning screw 4a penetratingclose to the opposite (bottom) guide wall, with a fine tuning screw 6 located at the guide centerline. 1

A further alternative arrangement is shown in FIG. 4, in which there is an offset first main tuning screw 4b and a centerline second main tuning screw 4c. Temperature compensation is obtained by the relative penetration of the two screws.

The offset capacitive tuning screw is preferably made of amaterial having a lower temperature coefi'lcient of expansion from that of the material of the waveguide. This results ina greater change in d with temperature, thedifferential changes in screw length and waveguide height allowing compensation to be achieved with reasonable gaps betweenthe screw and number of conditions are shown in FIG. 5. Curve A shows the behavior of a conventional resonant cavity constructed in 2 in. X in copper waveguide.

Curve B represents the corresponding behavior of a 4 GHz. evanescent cavity built in X-band copper guide of 0.9 in. X 0.4 in. with a 2 BA copper tuning screw, as in FIG. 1. The value of d (0.042 in.) is in the range where some compensation is possible and some incidental compensation probably occurs.

Curve C shows the behavior of a similar evanescent mode waveguide resonator to that of FIG. I, having a 2 BA centrally located capacitive tuning screw with the same value for d but made from silver-plated Swedish iron. Swedish iron has a lower coefficient of expansion 12Xl0"/C.) than copper (l6Xl0"/C.). v v A ln curve D, the 2 BA silver-plated Swedish iron screw has been located halfway between the guide center and the sidewall, as in FIG. 2. The value of d (0.028 in.) is then such I that overcompensation occurs. V

In curve E, a fine tuning screw (6 B of copper has een introduced at the center of the guide, as in'FIG. 3, and the main 2 BA tuning screw of silver-plated Swedish iron withdrawn slightly, the compensation is then nearly exact.

It should be emphasized that the amount of offset of the main tuning screw and the choice of its size, .i.e., its cross-sectional area, are determined largely on an experimental basis and that for example offsetting the screw halfway between the tion of evanescent H-mode waveguide for providing significant end-loading capacitance, said tuning screw is of a material having a lower coefficient ofexpansion than that of the material of said section of evanescent H-mode waveguide.

2. A temperature-compensated resonator, according to claim 1, wherein said waveguide section is made of copper and wherein said tuning screw is made of silver-plated Swedish iron. 

1. A temperature-compensated resonator comprising a section of evanescent H-mode waveguide and a single capacitive tuning screw on the broad longitudinal center line of said section of evanescent H-mode waveguide for providing significant end-loading capacitance, said tuning screw is of a material having a lower coefficient of expansion than that of the material of said section of evanescent H-mode waveguide.
 2. A temperature-compensated resonator, according to claim 1, wherein said waveguide section is made of copper and wherein said tuning screw is made of silver-plated Swedish iron. 